For plastic moulding, e.g. through injection moulding or through compression moulding, for die casting, and for the extrusion of metals, e.g. aluminum, magnesium and zinc, there are used tools (moulds and dies, respectively) made from tool steel. These tools are often very large and have cavities with very complicated designs.
In order for the tools to exhibit the desired performance and to have the desired working life, the tool steel has to satisfy a number of different features, depending on how and for what purposes the tool is to be used. Usually the stresses on the tools are high, and include mechanical as well as thermal stresses, and also various forms of wear. Basically, the tool steel should have a high and uniform hardness, even when in the form of bodies having large dimensions, while at the same time as it should have a sufficient toughness for the use in question.
Now, usually tough-hardening steels of type grade AISI P20 (0.35% C-0.4% Si-0.8% Mn-1.8% Cr-0.4% Mo) are used all over the world as a tool material for plastic moulding and for zinc die-casting. Such tool steels are usually delivered from the steel manufacturer in the tough hardened condition, i.e. hardened and high temperature tempered to a hardness level of about 33 HRC. The tools then are made from such steels and, the tools are usually also used in this hardened, tempered condition. In those cases when higher hardness is needed in the tool, which recently has become more and more common, the finished tool has to be rehardened and tempered, which gives rise to increased risk of cracking and dimension changes of the tool which are difficult to resolve. These tough hardening steels, in other words, have evident drawbacks, which cause problems for the steel manufacturer, as well as for the tool maker and/or tool user, namely:
The steels are complicated to manufacture, since they require specific intermediate annealing operations to be performed by the steel manufacturer to eliminate the risk of cracking during manufacture. The steels also require a finishing, full tough hardening operation.
The steels strongly limit the possibilities of, utilizing the higher hardnesses of the tools when required, and they therefore reduce the end user's flexibility in terms of obtaining appropriate tool features.
It is possible to improve the possibility of achieving desired hardness levels by adding alloying elements to the steel, which may give rise to so called precipitation hardening, i.e. increase of the hardness of the steel through a simple heat treatment operation (ageing). The AISI-standardized grade P21 steel having the nominal composition: 0.20% C-0.3% Si-0.3% Mn-4% Ni-1.2% Al, is an example of a tool steel of this type which has been long known.
A steel having the nominal composition 0.15% C-0.3% Si-0.8% Mn-3.0% Ni-0.3% Mo-1.0% Cu-1.0% Al (U.S. Pat. No. 3,824,096) is a considerably newer example of a similar type steel. In both cases aluminum, in the latter case also copper, is used as a precipitation hardening alloying addition. The combination of alloying elements of these steels, however, will cause the steels after cooling from high temperature (in the austenitic state), depending on dimension and cooling procedure, to have a structure consisting of hard martensite (&gt;40 HRC) or softer bainite/ferrite or mixtures thereof. Therefore such steels have to be tempered (aged) by the steel manufacturer and are usually delivered in the as aged condition in the hardness range of 35-40 HRC. The precipitation hardening effect moreover is comparatively weak in these steels, and hardness levels exceeding 40 HRC are practically not possible to achieve for these steels through precipitation hardening. Today no suitable low alloyed steels exist which can eliminate the above mentioned drawbacks of the conventional tough-hardening steels. Theoretically, the very high alloyed marageing steels and certain precipitation hardening stainless steels may have the desired properties, but these steels are too expensive for most technical fields of application.